surface finish

The scratches - as per my experience in lapping are due to

a) larger particle in tha paste

b) too thick or too thin paste

c) Excessive pressure on the surface

These are tactile scratches.

As far as visible scratces are there - they will exist. In lapping you can not get a mirror finish but what you need is a surface finish and matched surface and not the optical reflectivity isn't it ?

lapping always results in dull surface (due to surface erosion) and the scratches(dull ness) depend on the above facts and the lengeth of the stroke in lapping.

sir actualy .. w r doin buffin after.. lapping... but now our client.... is demanding for less scratch surface.. in lapping itself without anyother process.... the workpiece is mild steel and lap plate is cast iron... In a book i saw that the lap plate should be softer than workpiece.... so if i change the plate could i reduce scratche.... the plate that can be used other than cast iron r brass,bronze,aluminium.... wat s ur suggestion....?

thank you.... can i know ur name pls..?

Hi,

What process do you use, before the lapping? Are you milling the piece? If so, the scratches left by the milling may be the culprit, as they are too deep for lapping to remove in less than "forever." I would suggest that you use a fly cutter, for the final pass after the milling. The fly cutter will have to spinning very fast, it shout have a rounded tip ground into it, and the milling machine bed should have an automatic feed to run the piece under the fly cutter at a precise rate of cutting. The amount taken off should only be .002 to .004 of an inch, otherwise the fly cutter will get too hot and start gouging the metal. Also, use a lubricating spray - yes! it is messy, but just put up a plastic shield so you don't get drenched.

This should produce an extremely flat surface, which you should be able to lap in a reasonably short length of time. The lines created by the fly cutter will be much shallower than those caused by a milling cutter.

tom

Depending upon how much light you illuminate the object with, you may be seeing "sleeks" which are submicron diffraction type artifacts caused by the "fit" of the object being lapped and the lap. We used a colloidioal silica and a buff polyurethane lap to eliminate these while polishing synchrotron mirrors RA,1 nm.

Use a piece of glass as your surface plate and use lapping compounds that decrease all the way down to jeweler's russe, to give it a polished no scratch look if you have the time and patience.

First you must determine the source of the scratches: If they are naturals ( original finish deeper than the grinding and polishing) you are not using your course grit long enough to get a good finish;

Are they from compound contamination? If any (as in any) of a courser grit compound gets into the finer compounds it will make scratches that the finer compounds will not be able to remove. Every time you go to a finer grit you must be 100% clean of any impurities or larger grits than the intended one;

Impurities can act like larger grit in two ways I know of, scratches and galling. These can be caused by to large of pieces of base material in the lapping action;

Galling can also be caused by to little lapping paste between your laps or your paste drying out thus letting the lap and the work piece touch;

Impurities can be from poor work practices or supplier for the most part.

My grinding laps are harder than my work piece and my polishing laps are softer. The grit is progressively finer (of higher mesh) how fine depends on the ultimate finish desired and the material to be lapped. My polish laps are damp but my grinding laps are water cooled. Most of my laps are diamond charged. My polish laps can be of anything that does the job on the particular material.

This is the general rule but there are exceptions.

I polish soft metals and some harder metals with a set of buffing wheels.

Brad

The point raised by UV is right.

In case there is a abnormal raise in the face (machining step/ handling damage etc) - there may be a metal to metal contact between the table and the job piece. The dent on the job piece will scratch the table and table in turn will scratch back the job (pun intended)

This will happen due to break in the non EHD (i dont think lapping paste is EHD) film failure and resulting metal to metal contact.

In case o deep groovem the lubricant (oil) may be squuezed out from the accummulated paste in the groove, and tha almost solidified paste may scratch.

About 10 years or so back, we have faced that problem. In the Hyd Stat brg (approx 1.5 Tons each race) while lapping. There was a small dent on one of the surfaces. End of lapping stage one- bothe the components were not lapped but scrapped. that was the extent of damage.

Afterwards we changed the process by bringing the components to approximate shape by blue matching (with each other if heavy and/or curved , with granite surface plate if light and flat) After a sufficient degree of blue is achieved (check the scatter pattern, and the individual depths and peaks in contact/ noncontact) - we do not take a risk if it is more than 0.02mm in this stage and pattern is approximately equally spread and already covers 40 to 50% surface area) we start the progressive lapping.

If you want a mirror finish you need to use a polishing pad on the lapping plate. We run 2 lapping tables, the cast iron plate gives us the flat surface but with a dull finish. For the application where we need a mirror finish (because the customer thinks it is a better quality) we use a self adhesive thin silk pad on the lapping plate. These are supplied by the lapping machine maker.

I thank you for your piece of expertise.Will keep it in mind and look at the problem again.

come back and tell us how you solved it (it may help some of us later)

Ok ..